Omnidirectional components of a unidirectional dynamic microphone are controlled by resistance (resistance control). Bidirectional components of a unidirectional dynamic microphone are controlled by mass (mass control). In order to obtain omnidirectional components, the dynamic microphone includes an acoustic resistor disposed immediately behind a diaphragm, which leads to flat frequency responses of resistance components.
FIG. 6 is a longitudinal cross-sectional view of a typical related dynamic microphone unit. As illustrated in FIG. 6, a unit case 1 functions as a base of the microphone unit 200. The unit case 1 is composed of a magnetic material. The unit case 1 serves as an outer yoke, and as an inner yoke 2, which will be described below. The unit case 1 functions as a part of a magnetic circuit. The unit case 1 is a cylindrical member having open ends, and is provided with a step 12 on an inner circumference in the middle of the vertical direction. A space above the step 12 corresponds to an air chamber 9, while a space below the step 12 corresponds to an air chamber 11.
The air chamber 9 in the unit case 1 accommodates a magnetic circuit composed of the following magnetic circuit components.
A disk inner yoke 2 fixed into the air chamber 9 is in contact with the step 12 of the unit case 1, which defines the position of the inner yoke 2 in the vertical direction. The outer circumferential surface of the inner yoke 2 is in contact with the inner circumferential surface of the unit case 1.
A disk magnet 3 fixed upon the inner yoke 2 has a smaller outer diameter than the inner diameter of the air chamber 9 in the unit case 1. A disk pole piece 4 is fixed upon the magnet 3. The magnet 3, the inner yoke 2, the unit case 1, and the pole piece 4 are the magnetic circuit components. The top surface of the pole piece 4 is substantially flush with the top surface of the unit case 1. The outer circumferential surface of the pole piece 4 faces the inner circumferential surface of the top portion of the unit case 1 with a proper gap to define a circular magnetic gap.
The magnet 3 generates a magnetic flux returning to the magnet 3 through a magnetic circuit composed of the inner yoke 2, the unit case 1, the magnetic gap, and the pole piece 4. In other words, the magnetic flux traverses the magnetic gap. The outer circumferential surface of the magnet 3 and the inner circumferential surface of the unit case 1 define an air chamber 9 below the magnetic gap, the air chamber 9 having a larger width than that of the magnetic gap. The pole piece 4, the magnet 3, and the inner yoke 2 respectively have center holes 41, 31, and 21 having the same diameter. The center holes 41, 31, and 21 are in communication with the air chamber 11 defined in a substantially bottom half of the unit case 1.
The outer circumferential surface of the top portion of the unit case 1 is fixed to a cylindrical member 35 composed of a nonmagnetic material. The cylindrical member 35 has a projection edge 36 along the inner circumferential surface at the top. The projection edge 36 is fixed to the outer circumferential surface of the top portion of the unit case 1.
The projection edge 36 of the cylindrical member 35 has a plurality of vertical through holes 37. The holes 37 are covered by a thin-sheet acoustic resistor 18 fixed on the top of the projection edge 36.
The outer circumferential surface of the unit case 1 and the inner circumferential surface of the cylindrical member 35 define a circular space 38 below the projection edge 36. The space 38 is in communication with a space behind a sub dome 52 of a diaphragm 5, which will be described below, through the acoustic resistor 18 and the holes 37.
The cylindrical member 35 has a projection edge 14 along the outer circumferential surface at the top. The cylindrical member 35 also has a concentric projection 13 inside the projection edge 14, the projection 13 having a height lower than the projection edge 14. The top surface of the projection 13 is fixed to the outer peripheral edge of a diaphragm 5. The diaphragm 5 is a thin film composed of a material such as synthetic resin or metal. The diaphragm 5 includes a center dome 51 and a sub-dome 52 surrounding the center dome 51 that are produced by shaping a thin film of a synthetic resin or metal.
The center dome 51 is a partial spherical shell. The sub-dome 52 has a partially arc-shaped cross section and extends along the peripheral edge of the center dome 51. The outer peripheral edge of the sub-dome 52 is fixed on the top surface of the projection 13.
The diaphragm 5 is fixed at its outer peripheral edge of the sub-dome 52 on the top surface of the projection 13 as described above. This enables the diaphragm 5 to vibrate in response to sound pressure from received sound waves, in the anteroposterior direction (the vertical direction in FIG. 6) around the outer peripheral edge of the sub-dome 52 as a supporting node.
A voice coil 6 is fixed along a circular boundary line between the center dome 51 and the sub-dome 52 in the diaphragm 5. The voice coil 6 is formed by winding a fine conductive wire and by fixing it into a cylindrical shape. One end of the cylindrical voice coil 6 is fixed to the diaphragm 5. The voice coil 6 is disposed in the magnetic gap while the outer peripheral edge of the sub-dome 52 in the diaphragm 5 is fixed to the diaphragm 5 as described above. In this state, the voice coil 6 is separated from both the unit case 1 and the pole piece 4.
A protector 7 for the diaphragm 5 is fixed on the top surface of the pole piece 4. The top portion of the protector 7 has a partially spherical shape or a dome shape similarly to the center dome 51 of the diaphragm 5. The protector 7 faces the reverse of the center dome 51 having a certain distance therebetween.
The protector 7 limits the vibration of excessive amplitude of the diaphragm 5, which prevents the diaphragm 5 and the voice coil 6 integrated to the diaphragm 5 from being damaged.
The protector 7 also has a center hole 71. A space adjacent to the reverse of the center dome 51 is in communication with the air chamber 11 through the center hole 71 and the center holes 41, 31, and 21 of the pole piece 4, the magnet 3, and the inner yoke 2 respectively, the center holes 41, 31, 21 having the same diameter.
Adjacent to the obverse of the diaphragm 5, an equalizer 8 serving also as a protector for the diaphragm 5 is fixed, at its peripheral edge, to the projection edge 14 of the unit case 1. The equalizer 8 has a ceiling surface having a dome shape in the center. The ceiling surface in the center of the equalizer 8 keeps a predetermined distance from the center dome 51 of the diaphragm 5. The equalizer 8 has a plurality of holes 82 for transmitting sound waves from the exterior to the diaphragm 5.
The bottom end of the unit case 1 is closed by a cap 10 to define a relatively large air chamber 11 in the unit case 1. In the air chamber 11, an acoustic resistor 16 adheres to the bottom surface of the yoke 2. The inner circumference of the air chamber 11 of the unit case 1 supports the outer circumference of the acoustic resistor 16. The acoustic resistor 16 is composed of, for example, a thickly-layered unwoven fabric. The acoustic resistor 16 is disposed in a space adjacent to the reverse of the diaphragm 5. The space adjacent to the reverse of the diaphragm 5 is closed by the cap 10 to define the relatively large air chamber 11 adjacent to the reverse of the diaphragm 5, as described above.
The diaphragm 5 vibrates in the anteroposterior direction in response to a variation in the sound pressure from received sound waves. The voice coil 6 also vibrates in the anteroposterior direction in cooperation with the diaphragm 5. The vibrating voice coil 6 traverses the magnetic flux extending through the magnetic gap. The voice coil 6 traversing the magnetic flux generates electric power as audio signals in response to a variation in the sound pressure. The dynamic microphone unit 200 electro-acoustically converts the signals as described above. For example, audio signals are outputted from the both ends of the voice coil 6 wound along the reverse of the sub-dome 52 to the exterior.
In the dynamic microphone unit 200 such a configuration, the acoustic resistor 16 in the space adjacent to the reverse of the diaphragm 5 provides an acoustic resistance to generate the omnidirectional components described above. Any materials such as an unwoven fabric, a resin mesh, sponge, or a felt can be used for the acoustic resistor 16. Particularly, a felt is preferable for the acoustic resistor to generate an omnidirectional component. A felt has a relatively high acoustic resistance density, and thus can have a relatively large volume. The felt, therefore, tends to readily absorb sound waves.
FIG. 7 is a graph representing observed acoustic absorption of a felt. As a frequency increases, the acoustic absorption of the felt increases and exhibits a peak around 2000 KHz. The felt having such characteristics as described above is used for the acoustic resistor to generate an omnidirectional component of the dynamic microphone, which can effectively prevent a resonance due to the space adjacent to the reverse of the diaphragm 5.
Felt is made of primarily compressed animal fibers. The fibers easily fall off from the surface of the felt. The dynamic microphone including the acoustic resistor made of such a felt has a problem in that the fibers, which have fall off from the surface of the acoustic resistor of a felt, intrude into, for example, the magnetic gap. The intruded fibers remain on the voice coil in the magnetic gap. The fibers remaining on the voice coil inhibits the movement of the voice coil, which results in unsatisfactory acoustic characteristics of the microphone.
When the acoustic resistor 16 made of an elastic material such as a felt is fixed to the microphone unit component (to the inner circumferential surface of the unit case 1 in FIG. 6), a gap may be defined between them. The gap transmits sound waves as shown by the arrow in FIG. 6. In this case, the acoustic resistor 16 fails to serve as an acoustic resistance, and causing the malfunction of the dynamic microphone.
A possible countermeasure to the problem is sealing the contact point between the edge of the felt as an acoustic resistor 16 and the unit component supporting the edge using a sealing material 17 such as a sealant as shown in FIG. 6.
This countermeasure cannot efficiently provide an acoustic resistor having a flat acoustic resistance because the felt is made of primarily compressed animal fibers as described above.
Another possible countermeasure to the problem is adjustment of the compression degree of the felt using, for example, a screw in order to produce an acoustic resistor having an adjustable acoustic resistance. Such an acoustic resistor, however, requires a complex structure and complicated manufacturing process that involve individual adjustments of the acoustic resistances of individual acoustic resistors.
The dynamic microphone including the acoustic resistor in the space adjacent to the reverse of the diaphragm is described in Japanese Unexamined Patent Application Publication Nos. 2011-14990 and 2007-300267.